Typically in the manufacture and assembly of a shoe-component part, a shoe manufacturer often desires to bond a box toe or heel counter, or other stiffening material, into the shoe before molding the shoe to the last. At the present, shoe manufacturers employ a woven fabric material which has been impregnated, coated or saturated with a thermoplastic polymer, such as polyvinyl acetate, a styrene-acrylic copolymer, ethylene vinyl acetate, etc. The sheet material may be coated on one or both sides with a thermoplastic adhesive to provide for bonding of the sheet material to component parts of the shoe. In operation, impregnated sheet material is positioned between an outer shoe upper material and an inner shoe liner, and placed in a mold where it is subject to heat and pressure to effect bonding of the sheet material between the leather and the shoe liner.
The thermoplastic impregnated fabrics employed as shoe stiffeners are subject to a number of disadvantages in use. One disadvantage is that the sheet material is stiff when cool, but softens when exposed to higher temperatures; that is, when subject to a temperature above the thermoplastic or solidifying temperature of the thermoplastic polymer employed. Shoes which contain such stiffener materials, when exposed to excessive heat, such as when placed in a store window subject to the sun or stored at high temperatures, have the stiffener material become soft during such exposure or storage. A further disadvantage is that the impregnated material is often just as hard during its use and manufacture as in the final state. Such material is typically stiff and difficult to handle, stretch, conform, stitch and fabricate to the desired shape. In attempt to overcome some of the disadvantages, the impregnating compositions have been compounded with plasticizers to increase the flexibility of the sheet material; however, this compounding has not been wholly satisfactory. Additional ingredients have also been added to the formulation, such as fillers, clays and the like, in order to provide for an increase in solids content at a reduced cost.
Shoe-stiffening materials of the thermoplastic type are described in U.S. Pats. Nos. 2,734,289; 3,113,906; and 3,393,461.
Thermosetting resins have been suggested for use in impregnating fabrics for shoe-stiffening materials, but are not commercially used or acceptable. Melamine-formaldehyde-saturated textile materials which are activated by immersion in an ammoniumchloride solution have been used, but are not acceptable because of residual toxicity and skin irritation problems. Elastomeric latex formulations have also been suggested as impregnants (see U.S. Pat. No. 2,611,195). However, toxicity, skin irritation, inability to control the reaction within the shoe-manufacturing process conditions, cosmetic problems and other factors have prevented commercial acceptance of such thermosetting resins for shoe-stiffening materials.